Caprolactam and other lactams are widely used in the chemical industry as monomers for the preparation of homo- and copolmyers such as the well-known nylons. For a review, see the chapter "Lactams" by F. Millich and KV Seshadri in Cyclic Monomers, edited by Kurt C. Frisch, Wiley-Interscience publishers, New York, New York (1969), pages 179-311. Such lactam monomers can be polymerized by various mechanisms such as condensation, cationic and anionic polymerization processes. For maximum polymerization efficiency, particularly in anionic polymerization, the lactam monomer should be highly pure and virtually free of contaminants. If it is not, contaminants can interfere with the polymerization, catalyst and process thus severely limiting both the yield and molecular weight of the polymer. Anionic polymerization grade caprolactam should contain a minimum of contaminants and particularly a minimum of water. Water is a potent poison in anionic polymerization because it reacts with catalysts, such as alkyl lithiums, and terminates "living" polymer chains. Because of its polar nature and broad solvent power, caprolactam persistantly contains water, and freeing it of water to the desired polymerization grade levels is technically, and especially economically, a difficult task.
A number of techniques have been described in the literature for the purification and particularly the removal of water from lactams, such as caprolactam. See, for example, the previously cited "Lactams" pages 236-241. Specific examples include U.S. Pat. No. 3,170,592 which discloses a process for purification of caprolactam in benzene solution by successive counter current washings with aqueous lactam solutions. The impurities in the benzene solution pass into the aqueous solution and provide an organic lactam solution free of salts, acids and alkalies. U.S. Pat. No. 3,156,683 discloses purification of lactams by refluxing at reduced pressure while passing inert gas through the lactam to remove volatile purities and then distilling the purfied lactam. U.S. Pat. No. 3,145,198 discloses the purification of lactam by a series of steps comprising alkaline treatment, distillative reduction and oxidation. U.S. Pat. No. 2,786,052 discloses purification of lactams by passing hydrogen through an aqueous solution thereof in the presence of a catalyst, while U.S. Pat. No. 3,154,540 describes lactam purification by treatment with active metal preparations, such as the combination of zinc, dust and Raney nickle. British Pat. No. 1,530,357 describes purification of lactams by extraction with aqueous base of a toluene solution of the lactam. Crystallization provides purified lactam from the toluene solution. U.S. Pat. No. 3,755,305 describes purification of caprolactam by a combination of extraction and recrystallization combined with vacuum distillation. U.S. Pat. No. 3,676,431 describes a "conventional" process comprising extraction with an aromatic liquid such as benzene, followed by stripping and distillation. U.S. Pat. No. 3,792,045 discloses crystal extraction of a lactam with a solvent, such as pentane or decane, followed by distillation. There is nothing in this process relating to water removal and, indeed, water in the form of steam, is added at several stages without regard for the moisture content of the ultimate product. U.S. Pat. No. 3,904,609 describes treatment of a solution of lactam in a phenol-like solvent, followed by crystallization, wherein the crystals are washed in hexane, filtered and vacuum dried. U.S. Pat. No. 4,148,793 describes lactam purification by distillation in the presence of water and followed by water removal through distillation from the lactam. No recyling or regeneration of solvent is described. U.S. Pat. No. 4,239,682 describes extraction of a toluene solution of caprolactam. U.S. Pat. No. 3,839,324 discloses solvent distillation of raw lactam and crystal extraction with a nonaromatic solvent, followed by distillation including regeneration and recycling. No attention is paid to water content, but it appears that water or steam are added during the multi-stage process.
The abstract of Belgium Pat. No. 773,264 describes lactam purification with a hydrocarbon solvent such as hexane followed by vacuum drying or distillation under an inert gas.
A variety of organic chemicals are often desired for anionic polymerization or other processes, in a dry or anhydrous form. It is not surprising that a number of processes have been reported to accomplish such drying. U.S. Pat. No. 2,695,867 describes drying of an organic compound by azeotropic distillation with a hydrocarbon, such as benzene or hexane, but includes no mention of caprolactam. U.S. Pat. No. 3,575,818 describes a process for the production of absolute ethanol (that is, dry ethanol) using pentane, while U.S. Pat. No. 4,379,025 describes the removal of water from butylene oxime with an organic C7-9 carbon atoms solvent.
The need for absolutely dry anhydrous caprolactam has been recognized as shown by the previously cited "Lactams" and Kirk Othmer Encyclopedia of Chemical Technology, Volume 18, "Polyamides (caprolactam)," John Wiley & Sons NY, (1982), page 432. Therein it is stated: "Anhydrous caprolactam is produced and supplied for use in anionic polymerization processes" (p. 432). The subsequent discussion, however, does not indicate how such anyhdrous material is made or its water content is measured. For the purposes of this invention, anhydrous or dry caprolactam (and its homologs) is considered to contain less than 50 parts per million (weight/weight) water as measured by the Karl Fischer or Micheler's Ketone technique.